1. Field of the Invention
The present invention relates to a signal transmission circuit. The signal transmission circuit according to the present invention is used, for example, for the transmission of the signal for an exposure process in an electron beam exposing apparatus used in the production of semiconductor devices.
2. Description of the Related Arts
In a prior art signal transmission circuit for an electron beam exposing apparatus, a signal for deflection for an electron beam exposing apparatus is amplified by an amplifier, and the amplified signal is supplied through a node to a coaxial cable as a transmission line, and the signal transmitted through the coaxial cable is supplied to deflection electrodes connected to the coaxial cable. The deflection electrodes are constituted by a pair of electrodes to which positive and negative voltages are applied.
The internal conductor of the coaxial cable is connected to the deflection electrodes, and the external conductor of the coaxial cable is directly connected to the ground.
The electron beam is deflected electrostatically by the signal for deflection supplied to the deflection electrodes through the coaxial cable, and the deflected electron beam is used for exposing a wafer of a semiconductor. Stranded wires are sometimes used in place of the coaxial cable for the signal transmission line.
In such a prior art signal transmission circuit, however, there are problems in that the delay in the transmission of the signal because of the capacitance of the coaxial cable or the twisted wires occurs, and the signal is difficult to transmit at high speed.
That is, in the case where the coaxial cable with the following deflection electrodes is not connected to the node at the output of the amplifier, the voltage of the node is raised quickly in response to the rise of the output voltage of the amplifier, but in the case where the coaxial cable with the following deflection electrodes is connected to the node at the output of the amplifier, the voltage of the node is raised only slowly in response to the rise of the output voltage of the amplifier due to the capacitance of the coaxial cable, thus causing a delay in the establishment of the voltage and preventing the signal from being transmitted at high speed. The problem is similar in the case of the prior art twisted wire transmission circuit.
If a signal transmission circuit without the coaxial cable structure is used for reducing the capacitance, there are problems that the characteristic impedance of the transmission line is higher than that of the coaxial cable or the twisted wires, that the effect of the shielding is not sufficient, and that the signal transmission circuit is apt to be affected by external noise.